During the first half of the twentieth century, coal burning at power plants, factories, and homes filled the air over the Midwestern U.S. with pollution. “Smoke”—as air pollution was usually called—used to occasionally block so much sunlight that people were forced to carry lamps in the middle of the day. In some eastern cities, particulate levels likely exceeded 1,000 micrograms per cubic meter—about twice as high as they are on a bad air quality day in modern Beijing, now one of the most polluted cities in the world.

The problem was especially bad in Pittsburgh. The hills surrounding the city were filled with bitumen, a type of soft coal that released large quantities of sulfate-producing gases, soot, and other pollutants when burned. In 1866, an Atlantic Monthly writer visited Pittsburgh and reported: “The town lies low, as at the bottom of an excavation, just visible through the mingled smoke and mist, and every object in it is black. Smoke, smoke, smoke—everywhere smoke.” It was, he wrote, “like looking over into hell with the lid taken off.”

The photograph above, taken in 1906, shows Pittsburgh’s Strip District neighborhood looking northwest from the roof of Union Station. The Fort Wayne Railroad Bridge spanning the Allegheny River is partly visible on the left. The photograph below is a smoky street-level view of the corner of Liberty and Fifth Avenues in Pittsburgh in 1940.

In October 1948, a disastrous outbreak of air pollution occurred in nearby Donora, Pennsylvania. What meteorologists call a temperature inversion—warmer air moving above cooler air—triggered the crisis. The warm air functioned like a lid, trapping fumes from steel and zinc smelters and exposing the town’s 13,000 residents to extremely high levels of sulfate pollution. The inversion lasted just four days, but the health consequences were swift: twenty people died and 6,000 suffered serious respiratory problems. Something similar happened in London in December 1952, leading to thousands of respiratory problems and premature deaths.

The natural-color satellite image below offers both a modern and historical context. Captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite, it shows heavy pollution heading toward Beijing, China, on December 12, 2013. If satellites had been flying in the first half of the 20th century, it’s reasonable to assume that skies over the eastern United States or Europe would have looked something like this on some days.